1. Field of the Invention
The present invention relates to an optical film for use in small-sized displays of cellular phones, personal digital assistants (PDAs), etc. The present invention also relates to a lighting device using the optical film.
2. Description of the Related Arts
Recently, lighting devices incorporating light-emitting diodes (hereinafter abbreviated as “LEDs”) as their light sources have begun to be used because LEDs have long service life and can easily take chromaticity control as compared to fluorescent display tubes.
FIGS. 8A to 8C show a conventional example in which a side-light type lighting device incorporating light-emitting diodes (LEDs) is used as a backlight. FIG. 8A is a plan view, and FIG. 8B is a side view. FIG. 8C is a sectional view showing the path of light in the lighting device.
In this lighting device, a plurality of LED light sources 44 are disposed at a side edge of a rectangular light guide plate 26 so that light emitted from the LED light sources 44 enters the light guide plate 26 through the side edge surface thereof. The light guide plate 26 has a plurality of reflecting prisms 70 formed on the bottom surface thereof, and each of the reflecting prisms extends parallel to the side edge at which the LEDs are disposed.
As shown in FIG. 8B, a reflecting sheet 48 is disposed under the bottom of the light guide plate 26, and a prism sheet 76 is disposed over the top surface of the light guide plate 26. These components of the lighting device are housed in a holder 42 to illuminate a non self-emission display 50 (see FIG. 8C), e.g. a liquid crystal display, which is placed directly above the holder 42.
FIG. 8C shows the behavior of light in the lighting device having the above-described structure. Light 78 emitted from the LED light sources 44 is propagated while repeating bouncing between the top surface of the light guide plate 26 and the surfaces of the reflecting prisms 70 provided on the bottom surface of the light guide plate 26. As the light 78 repeats bouncing or reflection, the incident angle of the light 78 on the top surface of the light guide plate 26 decreases and eventually becomes smaller than the critical angle. Consequently, light exits from the top surface of the light guide plate 26. Light 79 emitted through the bottom surface of the light guide plate 26 is reflected on the reflecting sheet 48 and returned to the light guide plate 26. The exiting light from the top surface of the light guide plate 26 is refracted by the prism sheet 76 and led to the display 50.
It should be noted that in the embodiments described below, the same members as those stated above are denoted by the same reference numerals.
There have been proposed many techniques wherein a light-refracting sheet such as a prism sheet or a lens sheet is placed over the above-described light guide plate 26 to illuminate a non self-emission display (for example, see Japanese Patent Application Publication No. 2002-42529).
In general, however, the light from the LED light sources 44 cannot be said to have high directivity, as shown in FIG. 8A. Accordingly, there are many light components 72 and 74 traveling in directions not perpendicular to the extending direction of the reflecting prisms 70. Such light is undesirably scattered when exiting the prism sheet 76, as indicated by reference numeral 80 in FIG. 8C. Consequently, there are many light components that are not substantially perpendicular to the light exit surface of the light guide plate 26. For this reason, the brightness of the lighting device cannot be high.
FIG. 9A is a graph showing optical characteristics of a conventional lighting device arranged as shown in FIGS. 8A to 8C. FIG. 9B is a view for explaining coordinate axes used in the specification of the present invention.
As shown in FIG. 9B, coordinate axes are defined as follows: an X axis extends in the width direction (i.e. the vertical direction as viewed in the figure) of the light exit surface of the light guide plate 26; a Z axis extends in the longitudinal direction (i.e. the horizontal direction as viewed in the figure) of the light exit surface of the light guide plate 26; and a Y axis is perpendicular to the X-Z plane.
FIG. 9A shows the directivity characteristics of the conventional lighting device graphed based on the coordinate axes defined as stated above.
As will be understood from the graph of FIG. 9A, the conventional lighting device has the following directivity characteristics. For example, in the X-Y plane, light in the angle direction of 30° from the direction perpendicular to the light exit surface of the light guide plate 26, i.e. the Y axis direction, has an about 40% intensity of light in the perpendicular direction (Y axis direction). In the Z-Y plane, light in the angle direction of 30° from the direction perpendicular to the light exit surface of the light guide plate 26, i.e. the Y axis direction, has an about 10% intensity of light in the perpendicular direction. That is, the directivity in the X-Y direction, which is the width direction, is particularly low. Thus, the conventional lighting device suffers from insufficient directivity of light and is consequently limited in brightness.
FIGS. 10A and 10B are reference drawings provided to explain optical characteristics of lighting devices, in which the same coordinate axes as those in FIG. 9A are used.
FIG. 10A shows optical characteristics of a lighting device having ideal directivity. Both the characteristics in the X-Y and Z-Y planes provide light only in the direction perpendicular to the light exit surface of the light guide plate, that is, in the direction of θ=0, as shown by solid line 86.
FIG. 10B shows optical characteristics of a lighting device having no directivity. Both the characteristics in the X-Y and Z-Y planes undesirably provide a constant light intensity (about 30% of the light intensity obtained with the above-described ideal directivity) at all angles to the light exit surface of the light guide plate, as shown by the thick solid line 88.